The invention relates to a laboratory centrifuge according to the preamble of claim 1.
Laboratory centrifuges, particularly for the medicinal-pharmaceutical field, are known in many forms. They consist of a housing which can be closed at the top by a cover and within which is a centrifuge rotor which is connected to an electrical drive and which is suspended so that it can oscillate. The centrifuge rotor is provided in the peripheral region with a plurality of exchangeable receptacles which are intended for receiving a mixture of substances which are to be treated by the centrifuging process. These processes must be carried out in dependence upon the chemical-physical properties of the mixture of substances under pressure and temperature conditions which deviate from the ambient conditions. In general, this requires a special design of the centrifuge housing, particularly the mounting of the centrifuge rotor within a bowl whose interior space can be brought to the right temperature, in particular can be cooled.
In order to carry out a centrifuging treatment at reduced temperatures, the interior chamber of the said bowl is cooled down to the particular product-specific temperature, initially with the drive at a standstill, and with the actual centrifuging treatment only beginning after this temperature has been achieved.
From GB2150717A there is known a laboratory centrifuge which is set up to the effect that the centrifuging process is carried out independently of the speed of rotation of the rotor and in accordance with a constant temperature. For this purpose the bowl which contains the rotor is provided with a temperature sensor by means of which, in combination with a control device, a circulation of coolant is brought into use according to the deviation of the measured temperature from a reference temperature, with the object of maintaining a constant temperature within the internal chamber of the bowl.
However, depending upon the cooling of the internal chamber of the bowl and of the centrifuge rotor there results a series of problems in the region of the electrical drive, which can be traced back to the thermal effect due to the cold rotor body and/or parts of the bowl. Thus, the cooling of parts of the motor as a result of thermal conduction and radiation leads to the formation of condensation which can trigger significant consequential damage, for example corrosion effects in the region of electrical contacts, a corrosive seizing up of the movable bearings of the two bearings of the motor shaft etc., so that finally, depending upon the magnitude of the damage which has occurred, one has to reckon on functional breakdowns, with this leading for its part to expensive repair work.
These problems arise only when the centrifuge rotor is at a standstill, since when the rotor is running no condensation occurs because of the inherent heating of the electrical drive and a sufficient movement of air, even at low temperatures of the centrifuge rotor.
From US5631509 it is known to equip an electrical drive with thermostatically controlled heating elements, and indeed with the object of heating its winding system, namely to avoid the formation of condensation and the consequential problems arising from this. The thermostat is set up so that the heating elements can be effective only when the electrical drive is at a standstill and so that a heating is carried out until the achievement of a minimum, pre-set temperature step above the ambient temperature.
From US4195324 there is known a further electrical drive which is equipped with auxiliary heating to avoid the formation of condensation. The electrical drive includes a three-phase alternating current motor, wherein one phase is electrically powered for the purpose of increasing the winding temperature above the dew point temperature of the surroundings.
It is a feature of these known electrical drives equipped with a standstill heating for suppressing the formation of condensation that the heating power to be installed within the framework of its standstill heating is only related to the ambient temperature. Undercooling due to the use of the electrical drive within a total machine, and the danger of the formation of condensation connected with this, is not addressed by these prior art documents.
These problems arise only when the centrifuge rotor is at a standstill, since when the rotor is running no condensation occurs because of the inherent heating of the electrical drive and a sufficient movement of air, even at low temperatures of the centrifuge rotor.
It is the object of the invention to provide a laboratory centrifuge of the type first mentioned above in which, in a simple way, the occurrence of condensation within the electrical drive when the centrifuge is at a standstill as a result of a process-dependent cooling of the centrifuge rotor is suppressed and the consequential damage arising from that can be avoided. This object is achieved in a laboratory centrifuge of the type referred to above by the features of the characterising portion of claim 1.
It is an essential feature of the invention to provide standstill heating which is effective exclusively when the centrifuge is at a standstill and preferably only if the actual centrifuging process is preceded by a product-specific cooling. The heating power which is introduced through the standstill heating is in each case of a magnitude so that at no position within the electric motor does the temperature fall below a dew point, so that a thermal effect on motor components arising from a cooling of the centrifuge motor is compensated in each case. The standstill heating must take into account these requirements as well as the heating power and also surface and spatial effects. If these conditions are fulfilled, then any suitable, preferably electrical, heating system can be utilized. The standstill heating is connected with an overall control, by means of which the heating is automatically actuatable, as soon as the initially mentioned conditions arise, namely after a cooling of the centrifuge motor and of the internal chamber of the bowl which holds the rotor, with the electric motor at a standstill.
According to the features of claim 2, the standstill heating means provides a fixed heating power which is of a magnitude such that even under the most unfavorable conditions, i.e. the lowest possible temperature of the centrifuge rotor, there is a sufficient heating of the motor to achieve the object first set out above. Within this framework, for example, the internal chamber temperature of the motor can be maintained at about 60xc2x0 C. and the heating power to maintain this temperature can be determined.
According to the features of claim 3, the power supplied by way of the standstill heating can be variable, and indeed according to a measure of the internal temperature of the bowl or of some other measured temperature value which at least approximately matches the cooling of the centrifuge rotor and from which a perceptible cooling of the motor components arises. In this case, the heating power supplied by way of the standstill heating matches the actual cooling.
The features of claim 4 are directed to a technical realisation of the last-mentioned concept, and for this there is provided a temperature sensor for detecting a measured temperature value of the bowl which is transferred to a control circuit by means of which the heating power of the standstill heating is controllable. The temperature sensor for this purpose can detect for example the temperature of the metallic bowl wall from which there is a proportional thermal effect on the motor components.
According to the features of claim 5, the standstill heating is effected directly through the winding system of the stator and/or the rotor of the electric motor. This brings about first of all the advantage that for the technical realisation of the standstill heating no additional component needs to be added to the structure of the motor. On the contrary, use is made of items which are already present. The particular winding system and the particular winding systems are electrically powered in such a way that no rotational movement results, but only a heating which is of a magnitude to achieve the object described above, namely to avoid falling below a dew point within the motor. This can be achieved by a powering of the winding system according to an input voltage and an input frequency matched to these conditions. This is applicable independently of other electric motor principles, for example whether the motor is a DC motor, a single phase AC motor or a multi-phase AC motor. The standstill heating means can be connected into a control circuit by means of which it is activatable according to predeterminable conditions and a predeterminable heating power. This control circuit can be integrated into the control of the motor which is otherwise provided, which on the input side is powered only by a measured temperature value which indicates the temperature of the centrifuge rotor and of the bowl. Even with the control of the heating power, in this way use can be made to the greatest possible extent of the possibilities of an electrical drive which are already present, in relation to the voltage and frequency controls, and indeed the stator winding system and also the rotor winding system. This means that conventional laboratory centrifuges can be converted at comparatively little expense, thereby to be in accordance with the present invention.